A power tool with a safety device for the above purpose is previously described in SE 524579, wherein the over-speed safety device comprises three main components, namely a speed responsive actuating mechanism, a valve element, and a valve retaining means for maintaining the valve element in open position during normal tool operation. In the above referred over-speed safety device the valve element is a disc-shaped pivotally supported flap type valve, and the retaining means comprises a magnet. At normal operation speed levels the valve element is retained in an open position by magnetic attraction, but at increased speed levels the actuating mechanism forces the valve element out of contact with the magnet whereby the valve element is released and free to move towards closed position to thereby stop or substantially reduce the pressure air supply to the motor.
A problem concerned with over-speed safety devices of the above type is a less reliable function of the valve retaining means. It has turned out to be difficult to obtain an accurate and stable contact between the valve element and the magnet, which means that the retaining force on the valve element has varied individually between different power tools and that the release action of the valve element has been inconsistent. Unintentional activations of the over-speed safety device, for instance, would cause costly working process interruptions. Also the actuating mechanism of the known device for releasing the valve element has turned out to be less reliable and suffered from fatigue problems, which could result in a malfunctioning safety device.
A proper and reliable function of the safety device is of great importance for protecting people and equipment in the vicinity of the work site from injuries and damages in case of a malfunctioning speed governor and a resulting attainment of a hazardous high motor speed levels. In most cases safety devices of this type are provided at power grinders wherein the working implement attached to the motor output shaft is a grinding disc. At motor speed levels exceeding a maximum accepted speed limit there is a risk for explosion/disintegration of the grinding disc with severe damages and injuries of people as a plausible result.